Nydalen is a 8 min subway trip from downtown.

Park Inn is the cheapest solution, but the rooms are much smaller and the restaurant/breakfast area is not as nice. The location distance wise is good, but the area is not the best, especially at night (not dangerous though).

Both the radissons are in good locations, the Scandinavia is closer to western part of the city, while the Plaza is right next to the central station and subway. You can't go wrong with either, but my preference would be Plaza.